The purification of 1,2-dichloro-1,1,2,2-tetrafluoroethane is known in the prior art, as illustrated by Belgian Pat. No. 685,511. In this type of prior art, the content of 1,2-dichlorotetrafluoroethane in a mixture of the 1,1- and 1,2- compounds is increased by passing the mixture as a liquid or gas at 50.degree.-500.degree. C. over a halogen-exchange catalyst, preferably activated carbon or alumina. The life of the catalyst can be enhanced by conducting the reaction in the presence of HF or Cl.sub.2. This patent discloses that the 1,1-isomer is significantly less stable at room temperature than the 1,2-isomer, especially toward hydrolysis. This type of prior art does not contact the isomer mixture with hydrogen in the presence of a catalyst. Furthermore, in Example 1 of the Belgian patent, a very high contact time of 48 hours is used. A further deficiency of this type of prior art is that it may not be capable of producing the 1,2-isomer substantially free of the 1,1-isomer. In this regard, the best example of the Belgian patent shows increase of 1,2-isomer from 92 to 98%.
It is also known in the prior art to react a mixture of 1,2-dichloro-1,1,2,2-tetrafluoroethane, and 1,1-dichloro-1,2,2,2-tetrafluoroethane with hydrogen at a high temperature in the presence of a hydrogenation catalyst. This type of process is illustrated by German Offenlegungsshrift No. 2,822,471, which was laid open on Dec. 7, 1978. A counterpart of this publication is Belgian Pat. No. 867,285 which is cited in Chemical Abstracts 90:151557w. This type of prior art process is used to produce tetrafluoroethane from dichlorotetrafluoroethane. The German publication uses temperatures as low as 240.degree. C., and contact times of about 10 seconds for temperatures ranging from 295.degree. to 420.degree. C. Palladium on carbon is disclosed as a catalyst useful in that process with 2 to 5 weight percent palladium being on the carbon substrate. The proportion of hydrogen used varies, with Example 1 thereof showing a 2:1 molar ratio of hydrogen to organic feed material.
In none of this art and in none of the prior art of which we are aware, is there a process for producing 1,2-dichloro-1,1,2,2-tetrafluoroethane substantially free of the 1,1-isomer. Also, in none of this art is there a process requiring a very short contact time for producing the 1,2-isomer substantially free of the 1,1-isomer.